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Polymeric positive temperature coefficient thermistor and process for preparing the samePolymeric positive temperature coefficient thermistor and process for preparing the same description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20080186128, Polymeric positive temperature coefficient thermistor and process for preparing the same. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION
1. Field of the Invention This invention is related to a type of polymeric positive temperature coefficient thermistor and its method of preparation. 2. Prior Art of the Invention The contemporary electronic products have been developed in the direction of light, thin and small characteristics whereas the electrical design has become more complicated with many different functions. Because each connected electronic device may happen to have unexpected problems, in order to prevent damage to the electronic device from an unusual short electric pulse, usually a fuse wire or a thermistor with character of positive temperature coefficient is used to break or limit current to the device to achieve the goal of protection of the electronic device. Because a thermistor with character of positive temperature coefficient can not only prevent the trouble, it also has reversible application function with repeatability. Therefore, it is widely applauded in the industry. A thermistor with positive thermal coefficient can use calcinated ceramics as the substrate. For example, a Japanese open patent 2005-317780 disclosed a kind of crystalline type of thermistor device with positive temperature coefficient. It was made by the thermistor formed on the surface of calcinating ceramics. The major component of the calcinated ceramics system is titanium barium oxide but part of the barium is substituted with lead. A United States patent application US 2006/0132280 A1 disclosed a kind of electronic device with positive coefficient with the device comprising: a substrate constituted with many ceramics layers and electrode layers, in which the electrode layer separates the adjacent ceramics layers. The ceramic layers contain ceramic materials and at least part of the R/T characteristic curve of the ceramic material shows a positive temperature coefficient character. Although it can be used in a wide range of voltage and temperature, the ceramic material has many drawbacks. The ceramic material needs to be calcinated at a high temperature and the process consumes both a lot of energy and a lot of time. It also has pretty high electrical resistance. Therefore, it does not meet the energy saving requirements for portable electronics. A thermistor with positive temperature coefficient can also use a polymeric conductive composite as the substrate. This type of thermistor is usually called polymer positive temperature coefficient thermistor. Polymeric electric conductive composite is made by using polymeric material as the substrate material and the substrate material is blended with an electric conductive filler. Once the blended electric conductive filler in the polymeric material reaches the critical filling fraction, the obtained polymeric conductive composite shows an abnormal increase in resistance value at a specific temperature. This phenomenon is called positive temperature coefficient transition behavior. The polymeric material itself is an insulator. By blending with an electric conductive filler, it becomes conductive (it is equivalent to the electric conductivity of the electric conductive filler) without sacrificing the attributes of the polymeric material such as good processibility and low consumption of energy during processing. Moreover, compared with a metal and a semiconductor material, the polymeric electric conductive material has a relatively lower density. It can be selected according to the different substrate and the electric conductive filler and can also be selected with different volume fraction of the filler. All of these result in different electric characters. Therefore, they can be used in many different applications. For example, if some good electric conductive metal particles are used as the filler, or some fillers with semiconductor character (e.g. carbon black, titanium dioxide, tin chloride) are used, when the volume fraction of the filler approaches the critical volume fraction, the electric conductivity of the obtained polymeric electric conductive material is about 106˜109 ohm-cm, which can be used as antistatic packaging materials or surface paint for electro-static dissipation, ESD. When the filling volume fraction is lower than the critical volume fraction, because the electric conductive circuit channel can not be formed, the electric conductivity of the polymeric electric conductive composite is close to the insulator substrate material. However, it can still be used as high dielectric materials required for inductive antennas and capacitors with high capacity. The polymeric electric conductive composite with a high filler volume fraction still exhibits high electric conductivity (<103 ohm-cm). Therefore, it can be used as the electric conductor contact, thermistor, electric current protection, electromagnetic interference shielding, EMIS and other applications. In addition, the change of the physical conditions of the environment (for example, the change of the temperature, outside applied pressure, the change of the applied voltage, the frequency of the supply power, outer magnetic field and electric field) or chemical interference (for example, humidity and solvent vapor) can also lead to various changes of the electrical properties of the polymeric electric conductive composite. The previously mentioned polymeric electric conductive composite has many advantages and meets the requirements for the current electronics industry for the mainstream design for portable electronics with light, thin, short, small, low energy consumption and modulator grouped. The polymeric positive temperature coefficient thermistor using a polymeric electric conductive composite as the substrate has been widely used in the electronics industry. The polymeric positive temperature coefficient thermistor using a polymeric electric conductive composite as the substrate has been reported in many published patent references. For example, a Patent I236489 of the Republic of China disclosed a kind of polymeric positive coefficient thermistor with excellent electric properties. It used the low density and low thermal coefficient ceramic particles galvanized with metallic layer as the filler for the polymeric electric conductive composite. The polymeric electric conductive composite uses a resin as the substrate. The resin that can be used for this application can be polyethylene, high density polyethylene, polypropylene, ethylene/propylene copolymer, polyimide resin, poly (methyl methacrylate), polystyrene, and thermoplastic elastomer polymers. A Japanese open patent 11-31603 disclosed a positive temperature coefficient resistor device used for protecting the circuit and its preparation method. The positive temperature coefficient resistor device was made by the following method. A certain number of electric conductive particles (carbon particles) were dispersed into a polymer, e.g. polyethylene to form a premixture, then the premixture was heated and stirred, then a certain amount of ceramics with electric conductivity (which was doped with Y2O3 and BaTiO3 powder) was added into this premixture. Finally, the mixture was heated and pressed to yield the product. In addition, a paper titled “Investigation of Polymeric Thick Film Positive Temperature Coefficient Thermistor (II)”, published by professor WEI-KUO CHIN of the department of chemical engineering at National Tsinghua University of the Republic of China in 2003, reported a type of polymeric positive temperature coefficient thermistor, which included a polyimide thin film. The thin film was made from polyimide blended with silicon dioxide particles and the surface of the silicon dioxide particles was coated with nickel. However, it has been found that there have been many problems in designing a polymeric positive temperature coefficient material with polymeric electric conductive composites. For example, for the electric conductive filler part, the widely used carbon black in the electronics industry has low electric conductivity and thus can not be used in miniaturized semiconductor devices. Although they can provide relatively better electric conductivity, because their density is much higher than polymer the metal particles can easily cause precipitation problems of metal particles during processing. Therefore, the metal particles affect the reliability and stability of the product. As to the substrate part, if a thermoplastic resin such as polyethylene and polystyrene is selected, the substrate material can not be used in high temperature environments because of its limitation of thermal resistance. If a thermo-setting resin such as polyaminoformic ester and epoxy resin, is selected, although it has relatively good thermal resistance, but because its thermal expansion coefficient is much lower than that of thermoplastic material, it is necessary that the content of the electric conductive particles be exactly controlled in order to obtain a material with an obvious positive temperature coefficient transition behavior. In addition, the viscosity of the thermo-setting resin may increase during the hardening process and thus can easily cause poor dispersion among particles and precipitation problem, which causes inexact control of the repeatability of the product. Because the application of a polymeric electric conductive composite in thermistor is still immature, the structural properties and the dependence of the positive temperature coefficient transition behavior on the content of the electric conductive particles and the particle distribution are still not known. As for the selection of the polymer substrate, currently, it is still limited to some polymers such as polyethylene, high density polyethylene, polypropylene, polymethyl methacrylate, polystyrene, epoxy and silicone rubber. The industry requires a kind of polymeric positive temperature coefficient thermal sensitive resistor with excellent positive temperature coefficient transition behavior in the absence of electric particle precipitation. BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 shows one of the shapes of the polymeric positive temperature coefficient thermistor of this invention. FIG. 2 shows the characterization results of the positive temperature coefficient behavior of the polymeric positive temperature coefficient thermistor.
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